Urticaria is a common skin disorder involving mast cell activation and degranulation. Urticaria is classified according to its chronicity into acute and chronic forms. It may occur spontaneously or upon exposure to a physical factor. In the latter case, the urticaria is classified as a physical urticaria. Physical urticaria may be induced by mechanical and applied pressure, exercise, or exposure to cold, heat, sun, water, or vibration. The pathologic basis of physical urticarias in general remains unclear and a genetic basis for these disorders has not been previously elucidated. The purpose of this project is to investigate the mast cell dependent pathogenic mechanisms of physical urticaria, both to better understand how to manage urticarial inflammation and to explore the consequences of mast cell degranulation in human tissues. In these studies, adult and pediatric patients undergo standard challenge testing to verify their urticaria. Blood samples are obtained for the investigation of molecular and genetic pathways involved in the disease process. Following the clinical induction of urticarial manifestations, additional blood samples are collected to determine soluble mediators involved in pathogenesis. Photographic imaging studies are performed during challenge testing. Skin biopsies may be obtained prior to and following challenge testing and are analyzed for biochemical and histological markers. Since the inception of the physical urticaria protocol in 2009, we have enrolled over 154 patients including 25 healthy subjects. All patients safely underwent challenge testing based on their history. Blood samples and skin biopsies have been collected and stored for biochemical, molecular and, when applicable, genetic analysis. Mast cell degranulation was verified by skin biopsy. The majority of the patients were challenge positive to either cold-induced, cholinergic, dermatographism, solar or vibratory urticaria. We identified two large Lebanese families with vibratory urticaria, a rare condition in which sustained vibration against the skin induces both a localized hive and systemic manifestations such as facial flushing. The findings of acute onset of symptoms with concurrent peripheral histamine release in affected family members implicates mast cell degranulation in the pathogenesis. This was also supported by increased staining of tryptase, a component of mast cell granular contents, in post-vibration patient skin samples compared to controls. Through linkage analysis and whole exome sequencing we identified a missense mutation that segregates with vibratory urticaria in these kindreds but was absent from variant databases and 1000 ancestry-matched controls. The gene that harbors this mutation encodes an adhesion G-protein coupled receptor that was highly expressed in human mast cells with a unique ligand ubiquitously found in the skin. Patient-derived primary mast cells, when adhered through this ligand and vibrated, degranulate. Likewise, transfected human LAD2 mast cells expressing this mutation showed greater degranulation in response to vibration than control cells. The mutation is a missense substitution in ADGRE2 (Adhesion G Protein-Coupled Receptor E2, also known as EMR2). ADGRE2 is an adhesion receptor that plays a role in cell attachment, granulocyte chemotaxis, degranulation and adhesion, and the release of inflammatory cytokines. The substitution resulted in the replacement of cysteine with tyrosine at amino acid position 492 (p.C492Y). ADGRE2 receptor undergoes autocatalytic cleavage, producing an extracellular subunit that is non-covalently bound to a trans-membrane subunit. The variant likely destabilizes this interaction, sensitizing mast cells to IgE-independent vibration-induced degranulation. This marks the first identification of a genetic basis for a mast cell-mediated urticaria induced by a mechanical stimulus and suggests that ADGRE2 subunit interactions may also have implications for other diseases involving mast cells. Comments regarding a possible connection of our findings in ADGRE2 and vibratory urticaria to an earlier report of a familial cohort with prolonged swelling following vibratory stimulation were further highlighted. This report entitled, Vibratory Urticaria Associated with a Missense Variant in ADGRE2, was published this year in the NEJM(Feb 3, 2016) and was highlighted in a joint NIH/NIAID/NHGRI press release. In addition, it has been selected featured in JACI's News Beyond Our Pagessection. A follow-up letter entitled, Vibratory Urticaria and ADGRE2, highlighting a possible connection to hereditary vibratory angioedema, was also published this year in NEJM (July 2016).